The present invention relates generally to alternating current (AC) induction motors and, more particularly, to a system and method for determining the stator winding resistance of AC motors by way of a motor drive, for thermal protection of AC motors, improving motor control performances, and condition monitoring of AC motors.
The usage of motor drives in various industries has recently become more prevalent based on the increasing need for energy savings and control flexibility in motor operation. Based on these needs, improvements in motor control performance have become increasingly important. One factor of improved motor control performance is the accuracy of motor parameter estimation, which is of great importance to the overall control performance of motor drives. Among the plurality of motor parameters that might be estimated, such as stator and rotor resistances, stator and rotor leakage inductances, magnetic inductance, etc., stator resistance is the most difficult parameter to be identified because of its small per unit value. However, the accuracy of stator resistance estimation is essential to accurately determining a plurality of related motor parameters. For example, an accurate estimation of stator resistance allows for the further estimation of rotor/stator flux, rotor speed, air-gap torque, stator copper loss, and other similar parameters. The accurate estimation of stator winding resistance is thus beneficial for motor controls and is widely used in motor condition monitoring, fault diagnostics and prognostics, and instantaneous efficiency evaluation.
Another known use for the estimated stator winding resistance is for determining stator winding temperature, which can be used for thermal protection of the motor. Thermal protection is an important aspect in the monitoring of motor conditions, as the thermal stress on the stator winding is considered to be one of the main reasons for stator winding insulation failure. It is commonly assumed that the motor's life is reduced by 50% for every 10° C. increase in temperature above an acceptable stator winding temperature limit. Therefore, accurate monitoring of the stator winding temperature is beneficial for motor protection purposes.
Various methods for determining the stator winding temperature have been proposed to estimate the average winding temperatures from the stator winding resistances. Over the years, various stator winding resistance estimation methods have been proposed for different purposes. Generally, they are divided into three major categories: direct measurement methods, equivalent circuit-based methods, and signal-injection-based methods. Direct methods, such as the IEEE standard-118, give the most accurate stator resistance estimates, but have limitations and drawbacks due to the fact that resistance is only measured at a certain temperature and the resistance variations due to temperature changes are not considered. A further drawback of direct measurement methods is that the motor has to be disconnected from service to perform the required tests.
The equivalent circuit-based methods of Rs estimation use the motor current and voltage to calculate the stator resistance based on an AC motor equivalent circuit (i.e., a model of the AC motor). Such model-based methods are non-intrusive and can respond to changes in the cooling conditions but are generally too sensitive to motor parameter variations to provide accurate Rs estimation, due to the fact that the motor parameters may vary under different conditions, such as operating speed, magnetic saturation, etc. That is, the estimation error of model-based methods can be larger than 20%. Thermal parameter variation and the difficulty of thermal parameter identification may lead to further inaccuracy in model-based methods.
The signal injection-based methods for determining stator resistance inject a DC bias into the stator supply voltage and use the DC component of the voltage and current to calculate the stator resistance. In one DC signal injection method, a resistor in parallel with a transistor is installed in one phase of the motor, which leads to an equivalent resistance in the induction motor that is different when input current is positive and negative, thus producing a DC component. Although this approach can be accurate and robust to the variations in cooling conditions and motor parameters, it suffers from its intrusive nature, as an extra DC signal injection circuit needs to be installed in series with one of the motor leads. Additionally, due to the current limits of semiconductor devices, previous signal injection-based methods cannot generally be directly applied to motors beyond 100 hp.
It would therefore be desirable to design an accurate, non-intrusive method for determining stator winding resistance. It would further be desirable to use an existing device to inject the DC component for determining stator resistance, and accordingly, to estimate the stator winding temperature.